Artificial fertilizers (ammonium sulfate and urea fertilizer), which are inevitable in producing crops to support continued existence of the human race, are produced from ammonia. A technique for synthesizing ammonia by employing nitrogen and hydrogen as starting materials and by utilizing a catalyst made of primarily iron was discovered by Haber and Bosch. That technique (called a “Haber-Bosch process”) has been used as an essential technique to support the life of humankind up to now even after the lapse of about one century since the Haber-Bosch process was industrially completed in 1910's.
The Haber-Bosch process includes a step of direct reaction of a gas mixture of nitrogen and hydrogen to perform a reaction under high-temperature and high-pressure conditions of 400 to 600° C. and about 20 MPa to about 100 MPa by utilizing a doubly promoted iron catalyst, which is primarily made of Fe3O4 containing several weight percent of Al2O3 and K2O, and a step of separating ammonia produced through the reaction of N2+3H2→2NH3 by cooling the produced ammonia or absorbing the same with water. Even at present, such a technique is industrially used in a production process substantially in the same manner as that when it was initially completed.
On the other hand, there is a known catalyst using, as a transition metal element having ammonia synthesis activity at low temperature of 300° C. or below, one kind of elements selected from among Mo, W, Re, Fe, Co, Ru, and Os, or any one of combinations of Fe and Ru, Ru and Re, and Fe and Mo substantially in a metallic state (Patent Literature (PTL) 1). Ammonia synthesis methods using, as a catalyst, any of group 8 or 9 transition metals, e.g., Fe, Ru, Os and Co, have also been developed (PTLs 2 to 4). Methods using, particularly, ruthenium as a catalyst for ammonia synthesis have further been developed (PTLs 5 to 8). Moreover, ammonia synthesis methods using, as a catalyst, any of nitrides of group 8 or 6B transition metals and composite nitrides of Co and Mo, have been developed (PTLs 9 and 10).
In addition, a method of producing ammonia from nitrogen and water vapor through plasma contact by employing a catalyst containing, in a support material, a component that has catalytic activity and that is selected as at least one transition metal from a group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Ru, Os, Co, Rh, Ir, Ni, Pd, Pt, Mn, and Cu, is filed for a patent application (PTL 11).
Hitherto, in order to utilize an ammonia synthesis catalyst, e.g., Ru or Fe, with high efficiency, magnesia, alumina, graphite, ceria, etc. have been used as catalyst supports, and alkali metals, alkali metal compounds, alkaline earth metal compounds, etc. have been used as promoters.
When an acidic compound, e.g., alumina, is employed as the support, it has usually been required to add a large amount of compound, which serves as a promoter having a high electro-negativity, for the purpose of increasing an electron donating ability and providing a catalyst with high activity.
Meanwhile, there is a chemical compound, having a mineral name of “mayenite”, among calcium aluminosilicates containing CaO, Al2O3 and SiO2 as ingredients. A compound having the same type of crystal structure as a crystal of the mayenite is called a “mayenite type compound”. The mayenite type compound has a representative composition of 12CaO.7Al2O3 (hereinafter denoted by “C12A7”). It is reported that a C12A7 crystal has a unique crystal structure where two of 66 oxygen ions present in a unit cell, containing two molecules, are included as “free oxygen ions” in a space within a cage, which forms the framework structure of C12A7 (Non Patent Literature (NPL) 1).
After 2003, the inventors have clarified that those free oxygen ions can be replaced with various anions. In particular, all of the free oxygen ions can be replaced with electrons by holding C12A7 in a strong reducing atmosphere. C12A7 in which the free oxygen ions are replaced with electrons can be expressed by a chemical formula of [Ca24Al28O64]4+(e−)4 (hereinafter denoted by [C12A7:e−]). A substance containing electrons substituted for anions as described above is called an electride, and the electride is featured in having a good electrical conductivity (NPL 2).
Furthermore, the inventors have found C12A7:e− that is an electronic conducting mayenite type compound, 12SrO.7Al2O3 that is a compound being of the same type as C12A7, a mixed crystal compound of C12A7 and 12SrO.7Al2O3, and a synthesis method thereof (PTL 12). An invention regarding a mayenite type compound in which Al is partly replaced with Ga or In is also filed for a patent application (PTL 16). Such a mayenite type compound is suitable as electrode materials requiring high-temperature heat treatment, e.g., a PDP protective film material and a charge electron injection material in an organic EL device. The inventors have further found that C12A7:e− containing conduction electrons at a concentration of 1×1019/cm3 or more and a compound being of the same type as C12A7 can be obtained by (A) a method of annealing a C12A7 single crystal at high temperature in vapor of an alkali metal or a alkaline earth metal, (B) a method of ion-implanting inactive ions into a C12A7 single crystal, or (C) a method of direct solidification from a melt of a C12A7 single crystal in a reducing atmosphere (PTL 13).
Moreover, the inventors have succeeded in obtaining C12A7:e−, which exhibits metallic electrical conductivity, by annealing a C12A7 single crystal in vapor of titanium metal (Ti), and have filed inventions regarding a production method of the C12A7:e− and usage thereof as an electron emission material for a patent application (PTL 14). The C12A7:e− exhibiting metallic electrical conductivity can also be directly synthesized in the form of powder by mixing CaCO3 and Al2O3 at a ratio of 11:7, heating the mixture at 1300° C., and further heating an obtained product in a vapor atmosphere of metallic Ca (NPL 3).
Since electrons included in C12A7:e− are loosely trapped within a cage of a positively charged framework structure, those electrons can be taken out to the exterior by applying a voltage or by employing chemical methods. On the basis of an idea that those electrons taken out to the exterior can be used in reductive reaction, the inventors have invented a method of producing secondary alcohol and diketone compounds by reducing ketone compounds by the electrons included in C12A7:e− and have filed the method for a patent application (PTL 15).